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When and why are database joins expensive?
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MySQL, Oracle, SQL Server, Sybase, postgreSQL,etc. care not the order of joins. I've worked with DB2 and it also, to my knowledge, doesn't care what order you put them in. This is not helpful advice in the general case
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Mysql: Select rows from a table that are not in another
... this only works as expected when none of the columns have NULL values. In MySQL NULL != NULL so every row that has a NULL value will be returned even if there is a duplicate row in the second table.
– Kyle Kochis
Apr 7 '15 at 2:49
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Rails 3: Get Random Record
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Thing.first(:order => "RANDOM()") # For MySQL :order => "RAND()", - thanx, @DanSingerman
# Rails 3
Thing.order("RANDOM()").first
or
Thing.first(:offset => rand(Thing.count))
# Rails 3
Thing.offset(rand(Thing.count)).first
Actually, in Rails 3 all example...
Remove Primary Key in MySQL
... following table schema which maps user_customers to permissions on a live MySQL database:
12 Answers
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Generate GUID in MySQL for existing Data?
I've just imported a bunch of data to a MySQL table and I have a column "GUID" that I want to basically fill down all existing rows with new and unique random GUID's.
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how to convert a string to date in mysql?
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As was told at MySQL Using a string column with date text as a date field, you can do
SELECT STR_TO_DATE(yourdatefield, '%m/%d/%Y')
FROM yourtable
You can also handle these date strings in WHERE clauses. For example
SELECT whatever...
How to delete duplicates on a MySQL table?
I need to DELETE duplicated rows for specified sid on a MySQL table.
25 Answers
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MySQL Cannot Add Foreign Key Constraint
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lol same. MySQL could use more precise error handling on this kind of stuff.
– dave
Apr 17 at 23:38
add a comm...
What scalability problems have you encountered using a NoSQL data store? [closed]
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I've switched a small subproject from MySQL to CouchDB, to be able to handle the load. The result was amazing.
About 2 years ago, we've released a self written software on http://www.ubuntuusers.de/ (which is probably the biggest German Linux community website)....
Delete with Join in MySQL
...ect_id = posts.project_id AND projects.client_id = :client_id;
BTW, with mysql using joins is almost always a way faster than subqueries...
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